The proposed research is aimed at understanding developmental changes in knowledge about the mind between middle childhood and adulthood, an age range that has not received much attention in this area. Studies will investigate developing conceptions of the distinctive features of different mental activities, and developing conceptions of the essential similarities and relationships among mental activities. The research focuses specifically on the development of concepts of mental activities that represent different ways of knowing, including but not limited to Memory, Comprehension, Attention Inference, Planning, and Comparison. Studies are designed to document the appearance of new concepts of mental activities with increasing age, and to examine developmental changes in the underlying organization of those concepts. This line of research has the potential for educational implications. Children's ability to construct a concept of Comprehension by differentiating Comprehension from Memory would have important educational consequences. Many school tasks require children to understand information in addition to simply remembering it. Children's eventual acquisition of complex but important learning strategies such as selfmonitoring of comprehension failures and successes may depend upon their being able to recognize and maintain a sensitivity to the differences between understanding information and simply remembering it. Subjects will be asked to rate the similarity, difficulty, etc., of exemplars of the above types of mental activities. Two techniques will be used to assess conceptual structure: Nonmetric multidimensional scaling (ALSCAL and INDSCAL), additive similarity trees, specifically extended similarity trees (ADDTREE and EXTREE) and pathfinder networks (PFNET). These three techniques are examples of the two families of models used to represent proximity data: spatial models, and hierarchical and non- hierarchical network models, respectively.